This invention relates to articles coated with an insulating ceramic barrier, and, more particularly, to such articles having a substrate, a platinum-group metal layer over the substrate, and a ceramic top coating.
In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is burned, and the hot exhaust gases are passed through a turbine mounted on the same shaft. The flow of gas turns the turbine, which turns the shaft and provides power to the compressor. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forwardly.
The hotter the exhaust gases, the more efficient is the operation of the jet engine. There is thus an incentive to raise the exhaust combustion gas temperature. However, the maximum temperature of the combustion gases is normally limited by the materials used to fabricate the turbine vanes and turbine blades of the turbine, upon which the combustion gases impinge when they are at their hottest temperatures. In current engines, the turbine vanes and blades are made of nickel-based superalloys, and can operate at temperatures of up to 1900-2100 F.
Many approaches have been used to increase the operating temperature limit of the turbine blades and vanes to their current levels. The composition and processing of the materials themselves have been improved, and physical cooling techniques are employed.
In another approach, a thermal barrier coating system is applied to the turbine blade or turbine vane component, which serves as a substrate. The thermal barrier coating system includes a ceramic thermal barrier coating that insulates the component from the hot exhaust gas, permitting the exhaust gas to be hotter than would otherwise be possible with the particular material and fabrication process of the component. An additional metallic layer called a bond coat is placed between the substrate and the thermal barrier coating to aid in adhering the ceramic thermal barrier coating to the substrate and to protect the substrate against contact with the exhaust gases and against oxidation.
In a variation of this approach, it is known to use a layer of platinum or other platinum-group metal instead of the bond coat, between the substrate and the thermal barrier coating. Such a structure has several advantages, including reduced interdiffusion between the bond coat and the substrate, enhanced formation and sustainability of a protective aluminum scale on the substrate, reduced cost, and maintainability of substrate mechanical properties. However, there remains opportunity for improvement of protective systems using a layer of a platinum-group metal and a ceramic thermal barrier coating. For example, spallation of the protective system can occur during operation of the protected component in extreme environments such as that of a gas turbine, leading to failure of the protective system.
There is an ongoing need for improved protected superalloy articles, and methods for their preparation, which attain long operating lives at elevated temperatures. The present invention fulfills this need, and further provides related advantages.
The present invention provides a method for preparing an article protected by a ceramic layer, and the article itself. The articles are operable at high temperatures because of the presence of an adherent thermal barrier coating, and have excellent resistance to spallation loss of the thermal barrier coating during repeated heating and cooling cycles such as those experienced in a gas turbine engine. The components are also resistant to degradation by environmental attack. These advantages are obtained even though the conventional bond coat is omitted. In one embodiment, the approach of the invention permits the rare earth/yttrium content of the substrate to be reduced or eliminated.
In accordance with the invention, a method for preparing a coated article comprises the steps of furnishing an article substrate, preferably a nickel-base superalloy, having a free sulfur content of less than about 1 part per million. The method further includes depositing a platinum-group metal layer over the article substrate, and applying a ceramic coating over the platinum-group metal layer.
The substrate article with low free sulfur content can be furnished in a variety of ways. The base metal can be selected to have a low free sulfur content. The composition of the base metal can be modified to result in a low free sulfur content. In one approach, a sulfur-scavenging element such as yttrium, hafnium, or zirconium is provided in the base metal in an amount sufficient to reduce the free sulfur content to less than about 1 part per million (ppm).
In another approach, a conventional high-sulfur base metal can be provided. The base metal is contacted to a reducing gas to remove the total, and thence free, sulfur to the required low level. For example, the base metal can be contacted at elevated temperatures to hydrogen or a hydrogen-containing gas that desulfurizes the metal.
The layer of platinum-group metal, preferably platinum, is deposited over the substrate having low free sulfur content. The layer is preferably from about 2 to about 10 micrometers thick, most preferably from about 4 to about 10 micrometers thick. The ceramic coating, preferably yttria-stabilized zirconia, is deposited over the layer of platinum-group metal. Both the layer of platinum-group metal and the ceramic coating are also low in sulfur, preferably less than 1 ppm.
The adverse consequences of a high sulfur content upon a protective system having a layer of a platinum-group metal and a ceramic coating have not been previously recognized. Consequently, in many instances the sulfur content of the underlying substrate upon which the protective system is deposited has not been reported. It may not be concluded from the absence of reporting of the sulfur content that the sulfur content is zero or otherwise less than about 1 part per million. Instead, in such situations it may be concluded that the sulfur content is likely in the typical range of about 5 to about 30 parts per million by weight, and that the sulfur content was not reported because there was no realization of its significance in this low concentration range.
The coated article of this type can be used in high-temperature applications in severe environments. A preferred application is as a gas turbine blade or vane, but the invention is not so limited. Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.